As you can see I use styrene to block everything off. This I secure first with 2 part 5 minute epoxy. I then use the casting resin. I hear about hull flex all the time. I will tell you this, if you cast the resin properly it will actually be stronger than the thin aluminum some of these companies use for their lower hulls. I get NO lower hull flex once I have done my resin and that is the really only important part that you want rigid all the time.

you only need to make it deep enough to come to the top of the motor mounts. you leave enough sticking up to mount your gears...